Education:

Diploma, Chemical Engineering, National Technical University of Athens, Greece (1981)

M.S., Chemical Engineering, University of Illinois at Urbana-Champaign (1983)

Ph.D., Chemical Engineering, University of Illinois at Urbana-Champaign (1986)

Research Interests:

Dr. Economou’s research focuses on plasma science and engineering as applied to etching and deposition of thin solid films for microelectronic device fabrication, nanotechnology, plasma medicine, and surface modification of materials. The projects described below are in collaboration with Prof. V. M. Donnelly.

The electron energy distribution function (EEDF), as well as the energy of ions bombarding the substrate (ion energy distribution or IED) are crucial for controlling etching rate and selectivity in advanced plasma processes used in the fabrication of devices with features down to 10 nm. Particle-in-Cell simulations with Monte Carlo Collisions (PIC-MCC) are employed to simulate the spatiotemporal evolution of the EEDF and IED in capacitively- and inductively-coupled plasmas. Of special interest is the application of tailored voltage waveforms to control the profile of the IED. Simulations are complemented with experimental measurements using Langmuir probes as well as non-intrusive optical diagnostics (EEDF), and retarding field energy analysis (IED). Recently we developed a methodology to obtain nearly monoenergetic (tight energy spread) ions bombarding the substrate. This was achieved by pulsing the plasma power and applying a synchronous DC bias voltage during the afterglow.

In-Plasma Photo-Assisted Etching

While studying ion-assisted etching of p-type silicon in chlorine-containing plasmas near the threshold energy a new, important phenomenon was discovered: in-plasma photo-assisted etching. This mechanism was first discovered in mostly Ar plasmas with a few percent added Cl2, but was found to be even more important in pure Cl2 plasmas. Nearly monoenergetic ion energy distributions (IEDs) were obtained by applying a synchronous DC bias on a “boundary electrode” during the afterglow of a pulsed, inductively-coupled, Faraday-shielded plasma. Such precisely controlled IEDs allowed the study of silicon etching as a function of ion energy, at near-threshold energies. Etching rates increased with the square root of the ion energy above the observed threshold of 16 eV, in agreement with published data (see figure). Surprisingly, a substantial etching rate was observed, independent of ion energy, when the ion energy was below the ion-assisted etching threshold. Experiments ruled out chemical etching by Cl atoms, etching assisted by Ar metastables, and etching mediated by holes and/or low energy electrons generated by Auger neutralization of low-energy ions, leaving photo-assisted etching as the only plausible explanation. Experiments were carried out with light and ions from the plasma either reaching the surface or being blocked, showing conclusively that the “sub-threshold” etching was due to photons, predominately at wavelengths <1700 Å. The photo-assisted etching (PAE) rate was equal to the ion-assisted etching rate at 36 eV, causing substantial complications for processes that require low ion energies to achieve high selectivity and low damage, such as atomic layer etching. Under these conditions, PAE likely plays an important role in profile evolution of features etched in Si with chlorine-containing plasmas, causing the commonly observed sloped sidewalls and undesired microtrenching. On the other hand, PAE can be beneficial by promoting extremely high selectivity in plasma etching of nanopatterns where, under certain conditions, plasmon resonance (plasmonics) may also play a role.

Atomic Layer Etching (ALE)

Etching with atomic layer precision is a critical requirement for advancing nanoscience and nanotechnology. Current plasma etching techniques do not have the level of control or damage-free nature that is needed for patterning delicate sub-10 nm structures. In addition, ALE methods proposed in the past, based on pulsed gases with long reactant adsorption and purging steps, are very slow, even for etching extremely thin films. In this project, principles and techniques are developed for a practical method of etching surfaces, one atomic layer at a time, using a combination of pulsed plasma and monoenergetic ion bombardment. With this novel methodology it should be possible to obtain ALE at a substantially higher rate (~30X), compared to other methods. Plasma experiments and simulations are performed to understand the complex interaction between the pulsed plasma and the resulting ion energy distributions. Measurements of time-resolved ion bombardment energy and angular distributions are coupled with etching experiments including the effect of noble gas ion mass, and reactant (Cl, Br, I) mass and electronegativity on sub-surface lattice damage and etching with monolayer precision. Plasma and surface diagnostics are employed to measure product removal rate as a function of chemisorbed layer surface coverage and substrate damage.

Low Temperature Atmospheric Pressure Plasmas

Interest in low-temperature atmospheric-pressure plasmas is fueled to a large extent by realized and potential biomedical applications. For selected area exposure, so-called atmospheric pressure plasma jets (APPJ) are most common. The plasma generated by this source extends up to several cm from the end of the tube where it mixes with open air, making it ideal for treating specimens, including bacteria-covered surfaces, or living tissue. Although the jet appears to be continuous, it consists of periodic streamers or “bullets” that propagate at speeds of 10 km/s or more. This project is a combined experimental-simulation study of APPJs interacting with surfaces. A schematic of the experimental setup is shown in the figure below. Optical emission spectroscopy (OES), in a wide range of wavelengths (UV to near IR), is the main plasma diagnostic. We are developing a new OES technique to be able to probe the last 100 nm of gas near a surface. At the same time, we are employing a plasma transport and reaction fluid model to predict the spatiotemporal profiles of plasma species and electric field. The physics of bullet interaction with specimens is of particular interest. The fluxes of important species (e.g., O atoms and ozone in the case of He plasma gas in an O2 ambient) on the surface of the specimen are predicted for both insulating and conducting surfaces, and compared to data.

An atmospheric pressure plasma jet impinging on a quartz substrate.

Nanopantography

In nanopantography, standard photolithography, thin film deposition, and etching are used to fabricate arrays of ion-focusing micro-lenses (e.g., small round holes through a metal/insulator structure) on a substrate such as a silicon wafer. The substrate is then placed in a vacuum chamber, a broad area collimated beam of ions is directed at the substrate, and electric potentials are applied to the lens arrays such that the ions focus at the bottoms of the holes (e.g., on the wafer surface). When the wafer is tilted off normal (with respect to the ion beam axis), the focal points in each hole are laterally displaced, allowing the focused beamlets to be rastered across the hole bottoms. In nanopantography, the desired pattern is replicated simultaneously in many closely spaced holes over an area limited only by the size of the broad-area ion beam. With the proper choice of ions and downstream gaseous ambient, the method can be used to deposit or etch materials. Data show that simultaneous impingement of an Ar+ beam and a Cl2 effusive beam on an array of 950 nm dia. lenses can be used to etch 10 nm dia. features into a Si substrate, a reduction of 95X. Simulations indicate that the focused “beamlet” diameter scale directly with lens diameter, thus a minimum feature size of ~1 nm should be possible with 90 nm dia. lenses. Thus far we have been able to write holes with diameter as small as 3 nm using a 230 nm diameter lens (see figure below). Transfer of patterns defined by nanopantography using highly selective plasma etching of Si, with the native silicon oxide as hard mask, can improve patterning speed and etch profile. With this method, arrays of high aspect ratio (>5) nanofeatures were fabricated in silicon with no mask undercut. The ability to fabricate complex patterns using nanopantography, followed by highly selective plasma etching, was also demonstrated. We expect nanopantography to become a viable method for overcoming one of the main obstacles in practical nanoscale fabrication – rapid, large-scale fabrication of virtually any shape and material nanostructure. Unlike all other focused ion or electron beam writing techniques, this self-aligned method is virtually unaffected by vibrations, thermal expansion, and other alignment problems that usually plague standard nanofabrication methods. This is because the ion focusing optics are built on the wafer.

Interlocking "UH" logo developed using amplification by plasma etching of latent pattern produced by nanopantography. 80 of the 7.5 million lenses are shown. Thinnest line is ~ 13 nm.